To conserve limited resources, reduction of fuel consumption of cars has become a strong social demand in recent years, and it is known that reduction of rolling resistance of a car tire is effective for this purpose. Since the tread portion contributes to the tire rolling resistance as much as about 50%, it is believed effective to improve the tread portion in order to reduce the tire rolling resistance.
It is believed that the rolling resistance of car tires results from energy loss of the tire materials due to repetitive deformation during driving.
On the other hand, braking performance of the tire on a wet road surface (hereinafter referred to as "wet road braking performance") must be high to ensure car safety. In respect to the tire materials, the wet road braking performance is determined by energy loss caused by frictional resistance as a result of deformation of the tread rubber material that follows up micro-fine unevenness of the road surface when brake is applied to tire and the tire is thus allowed to slide over the road surface.
As described above, both rolling resistance and wet braking performance greatly result from the energy loss of the tread material. Accordingly, reduction of the rolling resistance resulting from the tread portion of the tire and improvement in the wet braking performance are generally contradictory to each other. To balance both performances at a high level has been extremely difficult using the heretofore known starting rubber materials.
To overcome these problems, various polymers have been proposed as novel rubber materials for the cap tread.
Among these proposals, British Pat. Nos. 1,166,832 and 1,261,371 disclose that a tire having excellent wet road braking performance could be obtained if the tire uses for the cap tread a rubber composition consisting of at least one member selected from the group consisting of essentially amorphous polybutadiene rubber having a large content of 1,2-bonding units (hereinafter referred to as "V.BR"), natural rubber, polyisoprene rubber, styrene-butadiene copolymer rubber and polybutadiene rubber having a small content of the 1,2-bonding units, or a blend composition of these members. As disclosed especially in Japanese Patent Laid-Open No. 104343/1980, it is known that when a rubber composition consisting of 25 to 75% by weight of V.BR having 65 to 90 mol% of 1,2-bonding unit and 75 to 25% by weight of natural rubber and/or isoprene rubber is used for the tread, the low rolling resistance and the wet road braking performance can be balanced at a high level while escaping from the above-mentioned contradiction.
When the rubber composition consisting of the above-mentioned starting rubber components is used for the cap tread, however, machinability during tire production and driving durability of the resulting tire during driving become inferior to those of a tire using a general purpose cap tread material. Hence, the following improvements are to be made. Namely, if an ordinary two-split mold is employed at the vulcanization step during production of the tire using the rubber composition consisting of the above-mentioned rubber components, a part of the block of the rubber composition of the cap tread is likely to crack (hereinafter referred to as "rubber block crack") when the tire is withdrawn from the mold, or the rubber composition that has flowed into a vent hole is apt to be cut at its intermediate part, thereby causing vulcanization troubles. Moreover, during turning under highly severe conditions or driving on the rough road, the cap tread is damaged by chipping. Hence, durability of the rubber composition is inferior to that of the general purpose cap tread material.